Conventionally, a bulk-type solar cell is typically manufactured by the following method. First, for example, a p-type silicon substrate is prepared as a first conductivity-type substrate, and, after removing a damage layer of the silicon surface, which is generated when making slicing from a cast ingot, for example, by a thickness of 10 μm to 20 μm with a few to 20 wt. % sodium hydroxide or sodium hydrogen carbonate, anisotropic etching is performed with a solution in which IPA (isopropyl alcohol) is added to a similar low-concentrated alkaline solution to form a texture so that a silicon (111) surface is exposed.
Next, the substrate is treated, for example, for tens of minutes at 800 to 900° C., for example, in a mixed gas atmosphere of phosphorous oxychloride (POCl3), nitrogen, and oxygen to form an n-type layer as a second conductivity-type impurity layer uniformly on the entire surface. A favorable electrical property of a solar cell is obtained by setting a sheet resistance of the n-type layer uniformly formed on the silicon surface to about 30 to 80 Ω/□. Thereafter, the substrate is immersed in a hydrofluoric acid solution to etch away glassy material (PSG) deposited on the surface during a diffusion process.
Next, the n-type layer formed in an unnecessary area on the back surface and the like of the substrate is removed. Removal of the n-type layer is performed by immersing the substrate, for example, in a 20 wt % potassium hydroxide solution for a few minutes after causing polymer resist paste to adhere to the light receiving side of the substrate in a screen printing method and drying it for protecting the n-type layer formed on the light receiving side of the substrate. Thereafter, the resist is removed by an organic solvent. As another method of removing the n-type layer on the back surface and the like of this substrate, there is a method of performing end-face separation by laser or dry etching at the end of the process.
Next, a dielectric film, such as a silicon oxide film, a silicon nitride film or a titanium oxide film, is formed on the surface of the n-type layer with a uniform thickness as a dielectric film (anti-reflective film) for preventing reflection. When a silicon nitride film is formed as the anti-reflective film, for example, the film is formed by a plasma CVD method under reduced pressure at 300° C. or higher using SiH4 gas and NH3 gas as raw materials. The refractive index of the anti-reflective film is about 2.0 to 2.2 and the optimum film thickness is about 70 nm to 90 nm. It is to be noted that the anti-reflective film formed in such a manner is an insulator, so that this does not function as a solar cell in a situation that only a front side electrode is formed on this film.
Next, with the use of masks for forming grid electrodes and bus electrodes, silver paste to be a front side electrode is applied onto the anti-reflective film in the form of the grid electrodes and the bus electrodes in the screen printing method, and is dried.
Next, back aluminum electrode paste to be a back aluminum electrode and back silver paste to be back silver bus electrodes are applied to the back surface of the substrate in the form of the back aluminum electrode and in the form of the back silver bus electrodes, respectively, in the screen printing method, and are dried.
Next, the electrode paste applied to the front and back surfaces of the silicon substrate is fired simultaneously for a few minutes at about 600° C. to 900° C. Consequently, the grid electrodes and the bus electrodes are formed as front side electrodes on the anti-reflective film, and the back aluminum electrode and the back silver bus electrodes are formed as back side electrodes on the back surface of the silicon substrate. On the front side of the silicon substrate, while the anti-reflective film is melted by a glass material contained in the silver paste, a silver material comes into contact with the silicon and is re-solidified. Therefore, conduction between the front side electrode and the silicon substrate (n-type layer) is secured. Such a process is called a fire-through method. Moreover, the back aluminum electrode paste also reacts with the back surface of the silicon substrate to form a p+ layer immediately beneath the back aluminum electrode.